A rotating electric machine consists of a stator and a rotor. The stator 3 of the electric machine is provided with grooves 3.1 in which conductors 4 are installed (FIG. 1). The conductors normally consist of several parallel sub-conductors 4.1 to facilitate the manufacture and to reduce losses caused by the skin effect. When an alternating current flows along a conductor 4, an alternating flux is generated through the loops formed by sub-conductors 4.1, which causes so-called circulatory currents in these loops. As a result, the currents of all the sub-conductors are not in the same phase, and their amplitudes are not equal. In other words, so-called differential-mode currents flow along the sub-conductors in addition to the common-mode current. Thus, the resistive losses of the whole conductor 4 are greater than in a case in which the same total current is evenly distributed in all the sub-conductors 4.1. This problem is particularly manifested in machines with a high input frequency. The extra resistive losses due to the circulatory currents cause heating of the winding and limit the power output from the machine.
The circulatory currents are eliminated, if the winding can be made in such a way that the total flux through the loops formed by the parallel sub-conductors 4.1 is zero. In large generators, the position of each sub-conductor 4.1 in the groove is designed in advance, and the winding is implemented by using so-called Roebel rods in such a way that the total flow of the sub-conductor loops is eliminated almost totally. In smaller machines, however, this is not a practical solution.
In smaller machines, the conductors may be divided into several parallel conductors, and the position of these conductors may be alternating in the grooves. This reduces the extra losses caused by the problem of circulatory currents but increases the amount of work in the production.
One known solution is to use a so-called Litz wire. The Litz wire consists of a large number of thin parallel wires that are braided or twisted. Such a Litz wire is illustrated by a sub-conductor indicated with the reference 4.2 in FIG. 1. The structure of the Litz wire eliminates efficiently the total fluxes passing through the loops formed by its internal wires; in other words, a similar current flows through all the strands of a single Litz wire. The optimum solution would be to make the whole conductor of a single thick Litz wire. In practice, the conductors must be made of several parallel Litz wires, because it is difficult to insert a single thick conductor in a groove.
The extra losses caused by the circulatory currents may also be reduced by the design of the grooves and by instructions relating to the production of the winding.